During selected engine operating conditions, combustion phasing may be retarded to reduce peak in-cylinder pressures. As such, peak cylinder pressures may be reduced to maintain in-cylinder pressures within a limit above which cylinder integrity can be affected. The retarded combustion phasing may include retarded ignition timing (to a sub-optimal timing) in spark ignition engines, or retarded fuel injection timing in compression ignition engines. Retarded combustion phasing may also be used during engine calibration when an exhaust emission control device (e.g., an after-treatment system coupled to a diesel engine) is not fully functional. The retarded combustion phasing is using therein to increase the exhaust temperature and reduce NOx.
The inventors herein have recognized that retarding combustion phasing to limit peak cylinder pressures can degrade engine performance, in particular full load performance. The retarded combustion phasing also results in higher fuel consumption and higher exhaust temperatures than may be possible with more advanced combustion phasing, resulting in the engine's torque and power output being limited. The issue may be exacerbated when the engine is operating with low energy density fuels such as biodiesel, or other oxygenated fuels.
Likewise, when retarded combustion phasing is used for expediting catalyst heating, there may be an increase in hydrocarbon (HC) emissions. If the combustion phasing retard is limited to control the HC emissions, there may be a resultant limiting of the level of exhaust temperature or enthalpy that can be provided by the combustion phasing (in other words, less than the desired amount of heating). Other calibration parameters, such as EGR flow, can be adjusted to provide a trade-off with exhaust temperature. However, the calibration may not be as efficient in engine systems having multiple exhaust catalysts. For example, if an upstream oxidation catalyst becomes partially or fully active before a downstream reduction catalyst, the engine may continue to be calibrated for reduced HC emissions, thereby limiting the exhaust temperature at a higher NOx level. Overall, engine performance is degraded and the engine may be rendered emissions non-compliant.
In some engine systems, in lieu of retarding combustion phasing, peak firing pressures and exhaust temperatures may be raised by increasing a fuel injector nozzle size. Therein, the larger nozzle size allows for a higher fuel flow through an injector, resulting in a larger amount of fuel being delivered into a cylinder per injection. However, the inventors have recognized that the additional hardware can increase engine costs. Furthermore, the increased nozzle size can also result in degraded fuel economy and engine performance.
In one example, some of the above issues may be at least partly addressed by a method for an engine, comprising: in response to predicted peak in-cylinder pressure for a planned main fuel injection into a cylinder occurring later than a threshold timing, splitting the planned main fuel injection into at least a first and a second injection, wherein the first injection is advanced relative to a timing of the planned main fuel injection. In this way, exhaust temperatures may be controlled while maintaining cylinder pressure within limits, improving engine efficiency.
As an example, a main fuel injection may be split into two or more injections to provide a cylinder pressure profile wherein the cylinder peak pressure is at a target pressure. In particular, if the originally planned main fuel injection has a peak cylinder pressure that is higher than the target cylinder pressure (that is, a defined peak cylinder pressure target for the given engine operating conditions), the main fuel injection timing may be retarded to limit the cylinder pressure. To recoup at least some of the loss in performance and enthalpy resulting from the combustion phasing retard, the main fuel injection may be split into at least two injections with the first injection occurring at a timing earlier than/advanced from the timing of the originally planned main fuel injection, and the second injection occurring at a timing equivalent to or later than the timing of the originally planned a main fuel injection. The timing of the split injections may be adjusted based on engine parameters such as engine speed, load, boundary conditions, and engine architecture. While advancing the timing of the first injection, a proportion of the total fuel provided in the first injection is increased such that a peak cylinder pressure resulting from the first injection is at a pressure, which is at or just below the peak cylinder pressure target (or limit). At the same time, the amount of fuel provided in the second injection is correspondingly decreased, to maintain the total fuel injection quantity of the originally planned injection (or with the injection quantity adjusted to meet user commanded engine torque). A timing of the second injection is retarded such that a peak cylinder pressure resulting from the second injection occurs as close as possible to the peak cylinder pressure of the first injection, while maintaining the cylinder pressure within the cylinder pressure limit. In one example, the split fuel injection may be used for a main fuel injection in each cylinder until an exhaust temperature is sufficiently high to heat one or more exhaust catalysts, such as an upstream oxidation catalyst and a downstream reduction catalyst. Additionally or alternatively, the split fuel injection may be maintained until the exhaust temperature is high enough to raise the turbine inlet temperature above a target temperature.
By using the split fuel injection when the peak cylinder pressure limits are reached or exceeded, engine efficiency is improved, and maximum torque is achieved while keeping exhaust temperature below the maximum exhaust temperature limit. In this case, splitting the main injection allows more fuel to be injected, thereby producing more torque without increasing exhaust temperatures. By using the split injection to expedite catalyst light-off, the injection timings of the split injection may be shifted to increase exhaust temperatures if they are below the maximum exhaust temperature limit. In this case, the spit injection can also be used to provide a main injection—post injection strategy.
In this way, a main fuel injection may be split into a first injection that is as advanced as possible, and a second injection that is as close to the end of the first injection as possible. The use of a split injection reduces the amount of combustion phasing retard required to maintain peak cylinder pressures within pressure limits. As such, this improves fuel economy and engine performance, particularly at high loads. In addition, the peak cylinder pressures of the first and second injections may be provided sufficiently close to each other such that exhaust temperatures are raised quickly, improving emissions performance and reducing turbo lag. As such, this allows for increased engine power even when operating with low energy density fuels. Furthermore, the increased engine performance can be achieved without costly changes to hardware, such as while using low flow injector nozzles.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.